1. Field of the Invention
This invention relates to a nonvolatile memory, and more particularly to an electrically erasable read only memory (EEPROM).
2. Description of the Related Art
In various kinds of recent electronic equipments, EEPROMs are used as initialization memories. Occasionally, in consumer market appliances such as televisions and cameras, data which needs to be frequently updated and initialization data which only occasionally reloaded after being once written are stored in a mixed fashion in a nonvolatile memory. For example, in a television, if the power supply is switched off, data of the last channel being viewed immediately before the switch off, i.e. channel information such as a receiving frequency, volume and degree of screen brightness, is stored in the memory so that the television will start operating in the same conditions as before when it is switched on the next time. Since such data (hereinafter called "last channel memory data") is stored whenever the power supply is switched off, the memory is reloaded repeatedly. On the contrary, initialization data for presetting to match a channel button with a frequency rarely needs to change after having been set up once, and the memory must be reloaded only several times at most.
FIG. 7 shows a U-shaped curve representing the reloading characteristic of an EEPROM. Initial failure, which results from various causes in the manufacturing process, can be screened by conducting tests; but random failure and wear-out failure would still be problems. In a random failure region, though its failure frequency is very approximately zero, a much better idea is needed to get it to be absolutely zero. In a wear-out region, since the frequency of reloading and failure frequency increase, measures would be required to increase the frequency of reloading as high as possible until it enters the wear-out region.
One of the conventional measures employs a so-called ECC (error correction code) system.
In the ECC system, when correcting a 1-bit error, for example, 4-bit parity data is needed for 1-word-8-bit data. If this method is applied to every word address, the chip size would be large to increase the cost of production. Similarlly to a television receiver, in the case where last channel memory data which need to be reloaded frequently and preset memory data which rarely need to be reloaded are stored in the memory in a mixed fashion, wear-out failure as shown in FIG. 7 need scarcely be considered for the latter, i.e., preset memory data, but only for last channel memory data. However, according to this conventional method, since the ECC system is used to add parity bits to every word address of EEPROM even if the last channel memory data occupies only several bytes, it requires many more memory bits than necessary, which thereby increases the cost of production.